1. Field of the Invention
The present invention relates to an apparatus and method of testing a semiconductor wafer on which chips regularly arranged in a matrix pattern and, more particularly, an apparatus and method capable of properly processing inking errors caused in the process of determining whether the chips are good or bad.
2. Description of the Related Art
Semiconductor devices such as ICs and LSIs must be tested several times in the manufacturing course to evaluate their electrical characteristics. A wafer test process is performed between a wafer forming process and an assembly process. The wafer test process is intended to apply an electrical test to each of the devices (or semiconductor chips) in a matrix pattern after preprocesses such as pattern etching and protective film coating.
A wafer test system basically includes a wafer probing machine (also called a wafer prober) and a tester. These components are connected via a measuring line. A test-complete-signal and a test-fail-signal are exchanged between the wafer probing machine and the tester in response to a test-start-signal on a test control line.
This wafer probing machine comprises an X-Y table movable along the X- and Y-axes, a wafer setting stage equipped on the X-Y table, and a probe card having a plurality of probes which are brought into contact with pads of each chip on the semiconductor wafer placed on the wafer setting stage.
When a test of a given chip is finished in a probing test, the X-Y table is moved in the X- and Y-axes and a next chip is tested. A large number of chips formed on the semiconductor wafer in a matrix are tested in the unit of a row in this manner. Chips which are discriminated as defective are marked with an ink.
As disclosed in preliminarily-opened Japanese Patent Application Sho 59-35441, the conventional apparatus causes chips which are determined as defective to be marked with ink through the inker and distinguishes defective chips from good ones depending upon whether they are marked with ink or not. The tester or prober also serves to count good chips on the basis of their test results.
In the case of the conventional apparatus, however, ink is splashed to adhere to good chips around a defective one when the defective chip is to be marked with ink, thereby causing inking errors. Ink once adhered to the chip cannot be usually eliminated. When inking error is caused, therefore, it will create trouble in processes followed, because those chips which have been found good by the test is marked with ink.
In order to prevent this inking error, the operator is asked after the wafer test to count those chips which have no ink marking by a manual counter, observing all of chips on the semiconductor wafer through a magnifier. In other words, the operator tries to find marking errors of the inker by comparing the number of no marking chips counted by the operator's eyes with the number of chips which have been found good by the tester or prober.
In the case of the conventional apparatus, therefore, the counting of no marking chips depends largely upon whether or not the operator is skillful. In addition, the operator is likely to get fatigued and make counting errors because he must count the chips on the wafer through the magnifier.
When the number of chips counted has counting errors, it takes a substantial time to find the inker (or marker) malfunctioning, and processes followed will be carried out for this time period according to information including a wrong number of good chips. This lowers the reliability of chips thus produced.
It has been therefore asked that the process of accurately counting only those chips which have no ink marking is made automatic.